The roles of the putative avian oncornavirus-induced tumor antigen (TSSA) as distinct from the virus envelope antigens (VEA) as relevant antigens and their relative importance in the immunological response of avian species to tumor induction have not been well defined. The notion that cellular immunity to TSSA is the dominant determinant in infection with ATV has been accepted mostly in the light of the lack of clear-cut evidence as to the role of other antigens or other immune mechanisms. There is little evidence that such a TSSA exists, much less that it is the sole or main functional determinant of the ultimate outcome of host infection with ATV. We therefore propose to define the relevant antigens, viral envelope antigens, tumor-specific surface antigens (VEA, TSSA) or others involved in tumor recognition in chickens and to determine the relative contributions of the cellular and humoral limbs of the chicken immune system in tumor growth and rejection in vivo as assessed by in vitro correlates of immunity. Recent work in our laboratory utilized two independent measures of cell-mediated immunity, leukocyte adherence inhibition and leukocyte blastogenesis, to address these questions. Leukocytes from RSV-injected birds responded to antigen preparations of infected cells containing VEA. There was little, if any, measurable response to any antigens which were not virion structural antigens or possibly precursors of such antigens. Humoral responses of birds infected with RSV and ALV were also measured by isotopic techniques, and these findings were confirmed.